Heretofore, there has been much work related to development of hydrophones for use in sonar towed arrays, fixed coastal or deep water arrays, torpedo guidance systems, or the like. Piezoelectric transducers have been developed for use with these systems but they have a number of problems. The towed array system has on the order of 250 channels, and each transducer consumes power on the order of 25 watts per channel, resulting in approximately 6 kilowatts of power dissipated in the towed array. Furthermore, piezoelectric transducers are low efficiency devices at low acoustic frequencies, on the order of a few hundred hertz or below, and they exhibit a non-uniform response at these frequencies. Additionally, the systems are very costly and have reliability problems due to their complexity. Piezoelectric transducers are enclosed in a sealed, water-tight housing containing an oil. The transducers are adversely affected by water and may become defective if immersed in water. Thus the seal must be almost perfect for the transducers to perform properly in a water environment.
Because of the above mentioned difficulties with conventional detection systems, there has been interest in the development of fiber optic phase-modulated transducers. However, such transducers are sensitive to environmental factors, such as temperature changes, particularly at low frequencies. The phase response of the fiber optic materials to a temperature variation of 1.degree. C. is larger than the response to a pressure change of 1 Pa (Pascal) by a factor of greater than 10.sup.6. Accordingly, temperature fluctuations and shifts are serious drawbacks to such phase-modulated systems.
Additionally, conventional hydrophone systems employ electrical conductors between the transducer elements and signal processor units, which result in electrical connection problems due to the deterioration of solder joints and loose connector pins. Crosstalk between adjacent channels and electromagnetic interference problems also affect performance of such conventional systems. Furthermore, many conventional hydrophone systems require the use of preamplifiers located at the transducer to provide sufficient signal strength for transmission to the signal processor. For an additional discussion of the drawbacks of conventional hydrophones see U.S. Pat. No. 3,831,137 for "Acousto-Optic Underwater Detector."
An article by S. K. Sheem et al. entitled "Single-mode Fiber-optical Power Divider: Encapsulated Etching Technique", Optics Letters, Vol. 4, No. 1, Jan. 1979 describes an optical coupler whose configuration is substantially the same as one embodiment of the present invention. The article particularly deals with the etching process required to prepare optical fibers and construct an optical coupler. The article neither discloses nor suggests the use of this coupler for the purposes of the detection of acoustic signals, or the like. In addition, only an optical fiber version of the coupler is disclosed. The present invention is not limited to optical fibers, but may employ integrated or planar optics therein.